Most 6.5" speakers are designed for infinite baffle installation... In other words, no enclosure needed. That said, it does need a solid, leak free, mount that isolated the front of the speaker from the rear of the speaker.
Also, aside from the materials used, there is no real difference between "marine" speakers and most automotive speakers. As long as the automotive speaker doesn't use any steel baskets of paper cones, it will work as well as a "marine" speaker. However, some insurance companies could deny a claim on a marine
Vessel IF the cause of damage could be proven to be related to the "non marine" product being used.
I think just about any JLAudio speaker will work fine in a boat they all have powder coated baskets and polypropylene cones.